A standard thermostatic mixing valve which is provided with an internal thermostat so that when set to a desired temperature it will automatically adjust itself to maintain this temperature is described in German patent document 3,530,812. It has a housing formed with separate hot- and cold-water input connections and with a tempered-water output connection typically carrying a faucet. A valve body in the housing can move between full-cold and full-hot end positions to admit hot and cold water in different ratios, depending on the desired setting which itself is established by an adjustment knob or lever. A thermostat, which term here means a device capable of changing position or shape dependent on temperature, can act on the valve body to move it so as to keep the temperature of the water downstream of the valve body constant. Normally this thermostat is provided downstream of the dual flow-control valve in a tempered-water chamber so it can respond to the temperature of the water immediately at the valve.
In my U.S. Pat. No. 4,767,052 I describe another such a valve having a main housing having hot- and cold-water inlets, compartments, and valve seats. A valve body is displaceable axially in the housing between one end position engaging and blocking only the hot-water seat and an opposite end position engaging the blocking only the cold-water seat. On moving between these end positions the valve body passes through intermediate positions permitting flow from both inlet compartments past the respective seats into the outlet compartment. A fitting body fixed in the housing and defining a wall of the outlet compartment is formed with a throughgoing hole opening into the outlet compartment. A thermostat in the outlet compartment is fixed with the valve body to a tubular stem engaged through and limitedly axially movable in the hole of the fitting body. A seal ring engages around the stem to seal the hole and a spring is braced between the housing and the stem outside the outlet compartment to pull the thermostat and valve body into solid engagement with the fitting body. A temperature-selecting mechanism engages via the stem with the thermostat and operatively engages via the thermostat with the valve body.
Such a valve typically has in addition a stop mounted on the housing and another stop mounted on the adjustment knob to limit the angular movement of this knob. Thus the maximum and/or minimum temperature setting can be established. This feature is of particular use when the valve is to be used in a public setting to avoid wasting hot water and injuring the users.
The knob simply normally sits on large nut that can rotate on the housing and that carries a pusher that is engageable with the valve body, typically via a force limiter. Thus the knob is fairly exposed.
With this arrangement the external structure of the valve is therefore fairly easy to damage or tamper with. Repositioning the housing stop can change the maximum and/or minimum setting for the temperature, and just general rough use can break the knob which is typically anchored at a single central point to the actuating nut. The exposed parts are also subject just in the normal wear-and-tear of cleaning to substantial abuse when in a public setting so that they are frequently broken or worn out.